Anywhere in a settings.TEMPLATE_DIRS directory:

# ./admin/<app>/change_form.html:
****************************************************

{% block extrahead %}
  {{ block.super }}
  <style>label b{font-weight:normal;font-size:0.75em;color:#888;display:block;margin-top:120px;}</style>
  <script type="text/javascript">
    
    django.jQuery(document).ready(function(){
        django.jQuery('#id_body').keypress(function(){
            django.jQuery('label[for=id_body]').html("Body:<b>Word count: " + this.value.split(" ").length + "</b>");
        });
    });
  </script>
{% endblock %}